Agricultural use of chemicals requires periodic soil analysis. Generally, chemical soil analysis for use in herbicide and fertilizer applications requires that soil samples be taken from various portions of the field and then submitted to a laboratory. One of the difficulties with the process is that it usually takes weeks to obtain the results of the soil test from the laboratory. Another of the difficulties is sampling error; that is, the concentrations of chemicals can vary from one portion of the field to another portion of the same field. At best, one obtains an average of the chemicals in the soil.
In addition for the need to determine the chemical residues from farming, one may need to determine the chemical residues from industrial processes such as military operations (i.e., chemical residues such as explosives, propellants, fuels, lubricants) and industrial manufacturing (i.e., chemical residues such as Volatile Organic Compounds (VOCs), and Dense Non-aqueous Phase Liquids (DNAPLs))
One of the problems with detection of the chemical residues in soils is that many chemical residues do not exhibit substantial fluorescence, thus hindering detection of their presence in subsurface soil. In general, a probe is inserted into the soil, and a light source located in the probe directs light through a window in the probe and into the soil where a portion of the light is absorbed by chemicals in the soil These chemicals fluoresce resulting in an optical emission different from the incident light.
One of the methods of determining chemical residues in soil is with a spectroscope. One spectroscopic method uses a scanning spectroscope which uses a narrow bandwidth filter which scans soil sample emissions throughout the emission spectrum while producing an electrical output from a single detector. A disadvantage of this system is the possibility that, during the scan, some spectral lines will not be present throughout the scan resulting in uncertain soil residue measurements of the soil sample emission spectra.
Another method is the use of a Fourier Transform Infrared spectroscope which simultaneously views the entire emission spectrum of the soil sample and then converts the measured output to a soil sample emission spectra which reveals the chemical residues in the soil.
Through the use of transient infrared spectroscopy (TIS), many of the chemical residues can be identified, the concentration levels measured, and a three-dimensional site characterization of the soil contaminants can be logged and plotted in real time. Thus, the TIS provides an in situ measurement of chemical residue in the subsoil.
The present invention provides a method and apparatus for use in agricultural and environmental remediation applications to enable TIS to determine both the concentration of farm chemicals in the soil and their spatial distribution. This knowledge allows application of precise quantities of corrective farm chemicals such as fertilizers and herbicides to areas of the field needing them and determination of the concentrations of contaminating chemicals from commercial activities.
In general, TIS is used to detect the chemicals present in a moving solid stream by measuring changes in emissions in the mid-infrared spectrum (400 to 4000 cm.sup.-1 wave number or 25 to 2.5 micron wavelength) of the moving solid. This portion of the spectrum is well suited for soil analysis since it can provide information on the kinds and quantifies of chemical residue occurring as a result of agricultural applications of fertilizers, pesticides and herbicides
One of the problems with TIS is the high absorption rams in the mid-infrared spectrum of most solids. If the sample being analyzed is thick, the high self-absorption rates produce an emission spectrum resembling a featureless, blackbody. To avoid the self-absorption problem occurring with thick samples, only a very thin sample is analyzed. In Transient Infrared Spectrometry, one measures the emission characteristics of a thin sample. TIS avoids the self-absorption problem by measuring thermal emission behavior of a thin sample of a larger body before the thin sample comes into thermal equilibrium with the larger body and begins to self-absorb like a blackbody.
During the conventional TIS process, the solid moves through the field of view of an infrared spectroscope and, as it does so, the solid is subjected to thermal energy by a jet of either hot or cold gas, a heated platen, a laser beam, or a UV or incandescent lamp. The thermal energy strikes the surface of the solid producing a thin hot or cold layer at the surface of the solid. As the thermal energy begins to dissipate, the thin layer of excited solid begins to thicken rapidly, but before the thermal emissions of the thickened layer can be measured, the solid is moved from the field of view of the infrared spectroscope and a thin, newly excited solid is moved into the field of view. As a result, the layer within the field of view of the spectroscope is constantly replenished and remains thin and thus not subject to thick sample errors.
If the solid sample is heated, a thin layer on the solid sample becomes an emission source independent of the bulk of the solid. As long as the excited layer on the solid remains thin, the emission self-absorption is at a minimum and the observed spectrum is of a thin sample.
If the solid sample is cooled, the thin layer acts as a thin transmission sample, and the bulk of the material beneath the thin layer radiates its blackbody spectrum through the cold layer. The spectroscope observes the featureless, blackbody spectrum of the solid sample with the structured transmission spectrum of the cold layer superimposed on the blackbody spectrum of the cold layer. Signal processing removes the blackbody spectrum, thus leaving the spectrum of the molecular constituents in the thin surface layer.
In the present invention, the necessary movement of the soil past the probe is obtained by either a forward motion of the probe through the soil or use of a rotating heat source within the probe.
The present process and apparatus permits a user not only to measure the chemical content and fertilizer deficiencies of the soil on-the-go, but also allows the person to use the on-the-go measurements to control of the mount of chemical metered onto the soil.